1. Field of the Invention
The present invention relates to a piezoelectric electroacoustic transducer such as a piezoelectric receiver, piezoelectric sounder, piezoelectric speaker, and piezoelectric buzzer, and more particularly, to a diaphragm of a piezoelectric electroacoustic transducer.
2. Description of the Related Art
A piezoelectric electroacoustic transducer has been widely used for a piezoelectric receiver, piezoelectric buzzer, or other suitable device. This piezoelectric electroacoustic transducer typically includes a unimorph type diaphragm which is constructed by adhering a circular metallic plate to one surface of a circular piezoelectric ceramic plate, wherein the outer peripheral portion of the diaphragm is supported in a circular case, and wherein an opening of the case is closed by a cover. However, since the unimorph type diaphragm obtains bending vibration by adhering a ceramic plate, having an outer diameter that expands and contracts, to a metallic plate which does not change in size in accordance with a voltage application thereto, the unimorph type diaphragm has a drawback that the displacement thereof, and thus, the sound pressure thereof is minimal.
Japanese Unexamined Patent Application Publication No. 61-205100, discloses a bimorph type diaphragm having a laminated structure including a plurality of piezoelectric ceramic layers. This diaphragm utilizes a sintered body obtained by laminating a plurality of ceramic green sheets and a plurality of electrodes, and then simultaneously firing them. These electrodes of the diaphragm are electrically interconnected via through holes provided at positions which do not restrain the vibration of the diaphragm. By constructing the bimorph diaphragm so that first and second vibrational regions thereof disposed in succession in the thickness direction vibrate in opposite directions, a larger displacement, and thus, a larger sound pressure than that of a unimorph diaphragm is achieved.
In the above-described bimorph diaphragm, however, in order to vibrate the diaphragm including, for example, three ceramic layers in a bending mode, it is necessary to interconnect one main surface electrode with one internal electrode via a through hole, to interconnect the other main surface electrode with the other internal electrode via a through hole, and further to apply an alternating voltage between each of the main surface electrodes and a corresponding internal electrode, as shown in FIG. 17 in the above-described publication. This requires a complicated interconnection between main surface electrodes and internal electrodes, and thus, increases the production cost of the bimorph diaphragm.
In addition, when the laminated body is being polarized, a voltage must be applied between an internal electrode, and top and bottom main surface electrodes. For example, where a diaphragm has a three-layered structure, as shown in FIG. 14 in the above-described publication, two through holes electrically connected to an internal electrode are connected to a connection electrode, and polarization is performed by applying a high voltage between the connection electrode and the top and bottom main surface electrodes. The conventional bimorph diaphragm, thus, has a drawback that it requires extending the internal electrode outside via through holes in order to perform polarization, which requires a complicated process such as the formation of the connection electrode.
To overcome the above-described problems, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer which eliminates the need for the interconnection between main surface electrodes and internal electrodes, and which enables construction of a bimorph diaphragm using a simple connection structure.
Further, preferred embodiments of the present invention provide a piezoelectric electroacoustic transducer in which the polarization process is easily performed.
A first preferred embodiment of the present invention provides a piezoelectric electroacoustic transducer including a laminated body formed by laminating two or three piezoelectric ceramic layers, main surface electrodes each provided on the top surface and the bottom surface of the laminated body, and an internal electrode provided between adjacent piezoelectric ceramic layers. In this piezoelectric electroacoustic transducer, all of the ceramic layers are polarized in the same thickness direction, and by applying an alternating voltage across the main surface electrodes and the internal electrode, the laminated body generates a bending vibration.
In the laminated body according to preferred embodiments of the present invention, when an alternating voltage is applied between the main surface electrodes and the internal electrode, the directions of the electric field occurring on a ceramic layer on the top and bottom surfaces are opposite to each other in the thickness direction. On the other hand, the direction of the polarization of every ceramic layer is the same with respect to the thickness direction. If the direction of the polarization and that of the electric field are the same, the ceramic layer will contract in the direction of the plane, and if the direction of the polarization and that of the electric field are opposite to each other, the ceramic layer will expand in the direction of the plane. Therefore, if an alternating voltage is applied as described above, for example, when the top ceramic layer expands, the bottom ceramic layer contracts, which causes the laminated body to generate a bending vibration. Since the displacement of the diaphragm is larger than that yielded by a unimorph diaphragm, sound pressure generated by this diaphragm is substantially higher.
In preferred embodiments of the present invention, since bending vibration is generated by interconnecting the top and bottom main surface electrodes and applying an alternating voltage across the main surface electrodes and internal electrodes, unlike conventional diaphragms, a complicated interconnection between the main surface electrodes and internal electrodes is not required. This results in simplification of the structure and reduction in the manufacturing cost.
In accordance with the first preferred embodiment of the present invention, the internal electrode is connected to an end surface electrode provided on an end surface of the laminated body, and an alternating voltage is applied across the end surface electrode and two main surface electrodes. Therefore, additional machining, such as the formation of through holes, is not required.
Further, in accordance with the first preferred embodiment of the present invention, preferably, the laminated body includes three ceramic layers, and the thickness of an intermediate ceramic layer is between about 50 percent and about 80 percent of the overall thickness of the laminated body. To increase sound pressure, the number of ceramic layers of the laminated body may be increased, but where the thickness of the laminated body is fixed because of resonance frequency, the lamination number cannot be freely increased.
In a three-layered laminated body, since there is no potential difference between the two internal electrodes, the intermediate layer does not contribute to a bending vibration, and only the top and bottom ceramic layers vibrate in a bending mode. The thinner the ceramic layer is, the larger the displacement thereof is. Accordingly, if the overall thickness of the laminated body is set to a constant value and the thickness of the intermediate layer is greater than the thicknesses of the top and bottom ceramic layers, the thicknesses of the top and bottom ceramic layers contributing to a bending vibration are relatively thin, which results in increased displacement. If the intermediate ceramic layer is too thick, however, the top and bottom ceramic layers will be too thin, which reduces the strength thereof, leading to a failure to yield a large displacement. Therefore, by setting the thickness of the intermediate layer to about 50 percent to about 80 percent of the overall thickness of the laminated body, a much larger sound pressure is achieved.
Moreover, in accordance with the first preferred embodiment of the present invention, preferably, the laminated body is constituted of a sintered body obtained by laminating two or three ceramic green sheets via an electrode film, and simultaneously firing the laminated green sheets, and then all of the ceramic layers are polarized in the same direction with respect to the thickness direction by applying a voltage across the main surface electrodes provided on the top and bottom surfaces of the laminated body. Alternatively, the laminated body may be obtained by laminating and adhering a plurality of ceramic plates which have been previously fired and polarized. This method, however, does not produce a thin laminated body, which results in decreased sound pressure. In contrast, laminating ceramic layer sheets via an electrode film, and simultaneously firing the laminated ceramic layer sheets produces a laminated body which is very thin, which results in an increased sound pressure. In addition, since the polarization direction of each ceramic sheet of the laminated body is the same, the polarization process does not require the application of a voltage across the internal electrodes and the main surface electrodes, unlike the conventional method. That is, polarization is achieved by applying a voltage across only the top and bottom main surface electrodes, which greatly simplifies the polarization process.
When accommodating the laminated body in a housing, and using it as a sounding body such as a piezoelectric receiver or piezoelectric sounder, the laminated body preferably has a construction in accordance with a second preferred embodiment of the present invention. When preferred embodiments of the present invention are applied to a piezoelectric receiver, the laminated body is preferably used in the frequency range other than a resonance frequency range in order to respond to a wide range of frequencies. Therefore, the laminated body has a structure wherein only one set of opposing sides of the laminated body are supported in a case, and wherein the other set of opposing sides are displaceably sealed by an elastic sealant, such that the displacement is attained, although the vibrational energy of the laminated body is relatively small.
Where preferred embodiments of the present invention are applied to a piezoelectric sounder, the laminated body is used in a resonance frequency range in order to respond to a high-volume sound at a single frequency. In this case, to produce a very large vibrational energy of the laminated body, the laminated body is constructed such that all four sides of the laminated body are supported in a case.
In either of these structures, the main surface electrodes and the internal electrodes of the laminated body extend outside the housing without using lead wires, and therefore either structures can be constructed as a surface-mounting type component.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following description of preferred embodiments thereof with reference to the attached drawings.